Re: [PATCH v2 30/31] nfsd: only allow filesystems that set EXPORT_OP_STABLE_HANDLES

Chuck Lever
2026-01-19T16:53:10Z

On 1/19/26 11:26 AM, Jeff Layton wrote:
> Some filesystems have grown export operations in order to provide
> filehandles for local usage. Some of these filesystems are unsuitable
> for use with nfsd, since their filehandles are not stable across
> reboots.
>
> In check_export(), check whether EXPORT_OP_STABLE_HANDLES is set
> and return -EINVAL if it isn't.
>
> Signed-off-by: Jeff Layton <jlayton@kernel.org>
> ---
> fs/nfsd/export.c | 15 +++++++++++----
> 1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/fs/nfsd/export.c b/fs/nfsd/export.c
> index 2a1499f2ad196a6033787260881e451146283bdc..bc703cf58bfa210c7c57d49f22f15bc10d7cfc91 100644
> --- a/fs/nfsd/export.c
> +++ b/fs/nfsd/export.c
> @@ -422,13 +422,15 @@ static int check_export(const struct path *path, int *flags, unsigned char *uuid
> if (*flags & NFSEXP_V4ROOT)
> *flags |= NFSEXP_READONLY;
>
> - /* There are two requirements on a filesystem to be exportable.
> - * 1: We must be able to identify the filesystem from a number.
> + /* There are four requirements on a filesystem to be exportable:
> + * 1: We must be able to identify the filesystem from a number.
> * either a device number (so FS_REQUIRES_DEV needed)
> * or an FSID number (so NFSEXP_FSID or ->uuid is needed).
> - * 2: We must be able to find an inode from a filehandle.
> + * 2: We must be able to find an inode from a filehandle.
> * This means that s_export_op must be set.
> - * 3: We must not currently be on an idmapped mount.
> + * 3: It must provide stable filehandles.
> + * This means that EXPORT_OP_STABLE_HANDLES is set
> + * 4: We must not currently be on an idmapped mount.
> */

Let's avoid the royal "we" here... It seems to mean something
different in each of these rules. How about:

1. The filehandle must identify a filesystem by number

2. The filehandle must uniquely identify an inode

3. The exported filesystem must provide stable filehandles

4. The requested file must not reside on an idmapped mount

> if (!(inode->i_sb->s_type->fs_flags & FS_REQUIRES_DEV) &&
> !(*flags & NFSEXP_FSID) &&
> @@ -442,6 +444,11 @@ static int check_export(const struct path *path, int *flags, unsigned char *uuid
> return -EINVAL;
> }
>
> + if (!(inode->i_sb->s_export_op->flags & EXPORT_OP_STABLE_HANDLES)) {
> + dprintk("%s: fs does not provide stable filehandles!\n", __func__);
> + return -EINVAL;
> + }
> +
> if (is_idmapped_mnt(path->mnt)) {
> dprintk("exp_export: export of idmapped mounts not yet supported.\n");
> return -EINVAL;
>

Reviewed-by: Chuck Lever <chuck.lever@oracle.com>

--
Chuck Lever